1. Field of the Invention:
The present invention relates to an apparatus for performing recording of information and/or reproduction of information for an optical disk while detecting a tilt angle of the optical disk by using a tilt sensor, thereby performing a tilt compensation control, and also relates to a tilt sensor, a tilt compensation method and a disk structure which are used for accomplishing such a purpose.
2. Description of the Related Art:
If an optical disk, onto/from which information is recorded/reproduced, is warped, then a light beam emitted from an optical pickup is not vertically incident onto a recording surface of the optical disk, thereby causing deterioration in the recording/reproducing signal. The deterioration becomes serious in a high-density optical information recording and reproducing apparatus using en objective lens having a large numerical aperture (NA). In order to prevent such deterioration, for example, Japanese Laid-Open Patent Publications Nos. 61-160844 and 4-328332 disclose an apparatus for performing optical recording of information and/or optical reproduction of information for the optical disk (hereinafter, referred to as an optical information recording and reproducing apparatus) in which a tilt sensor for detecting a tilt angle of the recording surface of an optical disk with respect to an optical axis of an optical pickup is provided. In these patent publications, while performing a tilt compensation control for angularly adjusting an optical pickup so that the output value of the tilt sensor falls within a predetermined range, the recording of information and/or the reproduction of information are/is performed.
A widely used tilt sensor includes: a light-emitting diode functioning as a light source; a photodiode having two light receiving sections (hereinafter, referred to as a two-divided photodiode) and functioning as a pair of light receiving elements; and an aspheric lens formed on the light-emitting side of the light source in order to substantially collimate the light emitted from the light source, all of these components being integrally formed so as to form the tilt sensor. The collimated light beam emitted from the tilt sensor is reflected by the recording surface of the optical disk. The reflected light is detected by the two-divided photodiode of the tilt sensor and the differential signal obtained from the outputs of the light receiving sections of the photodiode is used as a tilt compensation signal. In general, the gain of the tilt sensor is adjusted so that the output of the differential signal of the tilt sensor becomes zero when the recording surface of the optical disk is vertical to the optical axis of the optical pickup.
A conventional tilt sensor is disclosed in Japanese Laid-Open Patent Publication No. 2-218027, for example. FIG. 16 shows the configuration of such a conventional tilt sensor 101. The tilt sensor 101 is mounted on a flexible substrate 103 and is electrically connected with a tilt compensation circuit (not shown). through the flexible substrate 103. The flexible substrate 103 is attached and fixed on the upper surface of a metal leaf spring 104 so that the tilt sensor 101 faces an optical disk 102. The metal leaf spring 104 is fixed on a base 105 of the optical pickup by fastening a fixing screw 106, thereby fixing the tilt sensor 101 on the base 105.
In assembling the tilt sensor 101, after performing a zero-point offset adjustment for adjusting the gain of the tilt sensor 101 so that the differential output between the pair of light receiving elements becomes zero when the light reflected by an optical disk 102 in an ideal state (i.e., a standard disk having no warp) is received by the light receiving elements, the tilt sensor is fixed on the base 105. This adjustment can be performed by fastening or loosening an adjusting screw 107. By fastening or loosening the adjusting screw 107, the metal leaf spring 104 is deflected between the positions 104a and 104b indicated by the broken lines in FIG. 16, and therefore the tilt sensor 101 moves between the positions 101a and 101b. The tilt sensor 101 is attached to the base 105 after adjusting the angle formed between the tilt sensor 101 and the recording surface of the optical disk 102 within the range of .theta.1 to .theta.2 by deflecting the metal leaf spring 104 so that the differential output becomes zero.
By implementing the tilt sensor in such a configuration, the zero-point offset adjustment can be performed in which the outputs of the pair of light receiving elements can be equalized when the light reflected by the unwarped optical disk 102 is received by the pair of light receiving elements, and than the attachment angle of the tilt sensor 101 is adjusted so that the differential output between the pair of light receiving elements becomes zero.
Next, referring to FIG. 17, the positional relationship between light spots formed on the recording surface of the optical disk by the optical pickup and the tilt sensor will be described.
The optical disk 102 includes: an information recording region 102a in which pits or grooves ere formed; and mirror portions 102b which include no pits or grooves and ere disposed on the inner side and the outer side thereof. The information recording region 102a consists of: a read-in region 102c; a program region 102d; and a read-out region 102e. In general, a directory for managing the information stored in the program region 102d exists in the read-in region 102c, and therefore the optical pickup first moves to read the directory. The read-out region 102e is provided for preparing for the case where the optical pickup is moved out of the program region 102d by mistake. In such a case, the optical pickup reads the address of the read-out region 102e, thereby returning the optical pickup to the program region 102d. Therefore, the optical pickup usually moves within the read-in region 102c and the program region 102d.
The center of the light spot 110a formed by the light beam emitted from the tilt sensor 101 is deviated from the center of the light spot 105a formed by the light beam emitted from the optical pickup toward a direction substantially perpendicular to the radial direction B of the optical disk. It is desired that the center of the optical axis of the optical pickup accords with the center of the optical axis of the tilt sensor 101. However, in order to avoid e mechanical interruption among the tilt sensor 101 and the components of the optical pickup, such as the objective lens and an objective lens actuator, some deviation occurs. Therefore, the center of the light spot 110a is deviated from the center of the light spot 105a.
As also shown in FIG. 17, the light spot 110a is deviated from the direction perpendicular to the direction B to a slightly inner side with respect to the center of the light spot 105a so that the light spot 110a is actually located on the same track where the light spot 105a is located. This is because it is considered that a more precise tilt compensation can be performed if the center of the light spot 110a is located on the same track where the light spot 105a is located. However, since the deviation of the light spot 110a from the direction perpendicular to the direction B is very small, the light spot 110a can be regarded to be deviated from the light spot 105a in a direction substantially vertical to the direction B.
The typical diameter of the light beam emitted from the tilt sensor 101 becomes as large as 5 mm. Therefore, in the case of arranging the light spots 105a and 110a at such positions, when the light spot 105a is located on the innermost circumference of the read-in region 102c of the information recording region 102a as shown in FIG. 17, a part of the light spot 110a possibly enters the mirror portion 102b. Similarly, when the light spot 105a is located on the outermost circumference of the program region 102d of the information recording region 102a, a part of the light spot 110a possibly enters the mirror portion 102b.
Since the light beam which has been emitted from the tilt sensor 101 and reflected by the information recording region 102a is diffracted by the pits or the grooves of the information recording region 102a, the amount of the light received by the light receiving elements of the tilt sensor 101 becomes smaller as compared with the light beam reflected by the mirror portion 102b. Therefore, in the case where the light spot 110a formed by the tilt sensor 101 partially exists in the mirror portion 102b, the amount of the light received by one of the pair of light receiving elements of the tilt sensor 101 becomes different from the amount of the light received by the other light receiving element, so that a differential signal output is generated. If the tilt compensation control is performed under such a state, then some failure is likely to occur in the recording and the reproduction because the optical pickup cannot direct the light beam perpendicular the optical pickup.
In order to avoid this failure, as disclosed in Japanese Utility Model Publication No. 3-13684, the tilt compensation control is suspended immediately before e part of the light spot 110a enters the mirror portion 102b and the state is maintained as a state where the normal tilt compensation control is performed, or an interpolation is performed based on the tilt compensation control data stored beforehand in the optical information recording and reproducing apparatus.
On the other hand, a high-density and large-capacity information recording and reproducing apparatus which performs recording and/or reproduction for a phase change type optical disk has recently been developed.
As a rewritable optical disk for recording, reproducing and erasing a signal thereon/therefrom, a phase change type optical disk using a chalcogel compound as a material of the recording thin film thereof is known. In general, when the recording thin film material is in a crystalline state, the state is regarded as an unrecorded state, and a signal is recorded on the recording thin film by irradiating the recording thin film with laser light to melt and cool the material so as to change the material into an amorphous state. On the other hand, in the case of erasing the recorded signal, the temperature of the recording thin film is increased by irradiating the thin film with a lower-output laser light than the light used in the recording, and the thin film material is changed into a crystalline state.
As the recording thin film material, a material which is mainly composed of Te, In, Sb, Se or the like and whose phase can reversely change between the amorphous state and the crystalline state, or a material in which the phase change is reversibly caused between two different kinds of crystalline structures is generally used. Such a phase change type recording operation has an advantage in that an information signal can be overwritten by using a single laser beam as a recording instrument. That is to say, by modulating the laser output between a recording level and an erasure level in accordance with the information signal and irradiating the track on which information has already been recorded with laser light again, a new signal can be recorded thereon while erasing the previous information signal. The above-described phase change type optical disk is disclosed, for example, in Japanese Laid-Open Patent Publication No. 56-145530.
For an optical information recording and reproducing apparatus for recording and reproducing information onto/from a phase change type optical disk, a phase change type optical disk having a capacity of about 500 MBytes and a diameter of 130 mm or less is used. The causes no problems even when the warp of the optical disk optical system of such an apparatus is designed so that the tilt angle formed between the optical disk and the optical axis of the recording/reproducing laser beam causes no problems even when the warp of the optical disk is about 0.3 mm, and the margins for recording and reproduction are secured. Therefore, such an apparatus does not include a function of compensating the tilt angle between the optical disk and the optical axis of the laser beam.
However, the above-described optical information recording and reproducing apparatus has the following problems.
First, in the case of using the tilt sensor having a configuration shown in FIG. 16, the zero-point offset adjustment varies the distance between the tilt sensor and the optical disk. Specifically, by fastening or loosening the adjusting screw 107, the distance L between the tilt sensor 101 and the optical disk 102 is varied in the range of .DELTA.La to .DELTA.Lb. Such variation becomes one of the obstacles of reducing the thickness of an optical information recording and reproducing apparatus. In addition, if the gain of the tilt sensor is adjusted, then the sensitivity of the light receiving elements of the tilt sensor is adversely varied. Therefore, in view of this variation in the sensitivity, the gain adjustment width of the tilt compensation circuit (not shown) is required to be large.
Furthermore, in a large-sized optical disk having a diameter as large as 300 mm, e.g., a laser disk, the tilt compensation control can be performed in a sufficiently large region of the disk, whereas in a small-sized optical disk having a diameter of 130 mm or less, it can be performed only in an extremely small region of the disk. Moreover, in an optical disk with a large warp, the tilt angle gradually increases from the inner circumference to the outer circumference thereof. Accordingly, in the case where the tilt compensation control is not performed in the outer region of the disk as in the method disclosed in Japanese Utility Model Publication No. 3-13864, the tilt angle of an actual disk becomes different from the interpolated tilt angle of an optical pickup. The smaller the diameter of an optical disk is, the larger the influence of the difference becomes. Therefore, a failure is likely to occur in a small-sized optical disk having a diameter of 130 mm or less when the recording or the reproduction is performed. In addition, an optical disk reproducing apparatus cannot reproduce the information recorded in an optical disk unless the apparatus can read the directory stored in the read-in region of the optical disk, and such a case is regarded as a disk error.
On the other hand, in an optical information recording and reproducing apparatus for recording/reproducing onto/from a phase change type optical disk, a mechanism for detecting and compensating the tilt of the disk is not provided, as described above. However, if the recording capacity is increased by increasing the recording density, it is effective to perform a tilt compensation control even in a small-sized optical disk having a diameter of 130 mm or less. In a phase change type optical disk, the information is recorded and/or reproduced by utilizing the difference in the reflectance between the crystalline portion and the amorphous portion. However, this difference in reflectance adversely effects the detection of the tilt of the optical disk. That is to say, the amount of the light reflected by the optical disk is varied when the light spot formed by the tilt sensor passes the boundary between the crystalline portion and the amorphous portion. Therefore, if the tilt angle of the phase change type disk is attempted to be detected by the tilt sensor described above, the outputs of the pair of light receiving elements of the tilt sensor become different from each other even though the optical disk has no tilt. As a result, the tilt sensor outputs a differential signal in passing the boundary between the crystalline portion and the amorphous portion as if the optical disk had a tilt, and an error is consequently caused in the tilt compensation control.